Poor Wandering Ones

Most people are aware that birds migrate, and many are aware that some butterflies migrate—most notably the Monarch, whose annual trip to Mexico is the stuff of legend. Fewer people, however, are aware that some dragonflies migrate.

Most dragonflies probably spend their entire lives in the vicinity of the body of water in which they spent their larval phase. Not all. Amongst dragonfly aficionados, the most famous travelers are the Wandering Gliders (Pantala flavescens), those mustard-colored dragonflies that you see trying to lay eggs on cars in parking lots in mid to late summer.

“This dragonfly is the world’s most widely distributed dragonfly being found in the tropics as well as in temperate zones. It breeds on every continent except Europe and Antarctica and is found all around the world. This species flies almost constantly and drifts with the wind as it feeds. It flies thousands of miles over oceans and because of its characteristic gliding flight, its common name is the Globe Skimmer or Wandering Glider. It can be seen flying almost anywhere in open country, sometimes in great numbers. They rarely perch but at sea they have been known to land on ships far away from land.”[1]

Pitcairn Island stamp of the Wandering Glider

These dragonflies can wander. Pitcairn Island is a British Overseas Territory in the South Pacific famous for being the home of the descendants of the mutineers from the HMS Bounty of Cap’n Bligh fame. It is 3000+ miles from New Zealand. [2]

I have been putting off writing this note in the hope that I could produce my own photo of a Wandering Glider—alas, so far, no joy.

At the same time, I became aware of the “Dragonfly Swarm Project” run by the redoubtable Dragonfly Woman on her blog. Finally, her writing introduced me to the work of Dr. Martin Wikelski of Princeton, who has studied the migration of Green Darners (Anax junius) by gluing itty-bitty radio trackers to 14 dragonflies. This is what Dr. Wikelski et al found:

“On average, they migrated every 2.9 ± 0.3 days, and their average net advance was 58 ± 11 km in 6.1 ± 0.9 days (11.9 ± 2.8 km d–1) in a generally southward direction (186 ± 52°). They migrated exclusively during the daytime, when wind speeds were less than 25 km h–1, regardless of wind direction, but only after two nights of successively lower temperatures (decrease of 2.1 ± 0.6°C in minimum temperature).”[3]

Amazing stuff.

So, what do I know? Very little, actually, but that has never stopped me in the past. Here’s what I have on Anax junius in Central Pennsylvania.

Swarming dragonflies are very difficult to photograph. I have yet to succeed.

Latest sightings: 28 September 2008 and 1 October 2010–and therein lies a tale.

I generally don’t see any Green Darners past August. However, 28 September 2008, a storm had blown up the night before from the south. The next day was damp and chilly, temperatures in the mid-60s at best. Beginning around noon, the sun would occasionally peek out from behind a cloud. Every time it appeared, even for a minute, several large dragonflies would launch out of my weed patch to hawk around the crab apple tree. My backyard consists of lawn, a ½ acre weed patch, a bunch of scattered shrubs and trees, a small seasonal stream, and some outbuildings on a small clearing in the woods along a country road. It is not what I would normally consider Green Darner habitat.

Male Green Darner. It was a cool day, note the "mood ring" abdomen.

One thing to note in these pictures is the color of the male’s abdomen. Apparently, when they are cold, the normal bright blue becomes this darker purple—which, I suppose, helps to absorb heat. Assuming that they don’t like to be cold, that must be sort of like a built-in mood ring: blue butt—happy bug; purple butt—sad bug.

Female Green Darner

Almost exactly the same scenario occurred on 1 October 2010. Those are the only two times that I have seen Green Darners in my backyard.

Here are the maximum and minimum temperatures by day for the seven days ending 28 September 2008, as recorded at Carlisle (about 20 miles south):

Here are the maximum and minimum temperatures by day for the seven days ending 30 September 2010:

It raises some interesting questions:

Is it possible that migrating dragonflies “fall out” when confronted with contrary weather conditions?

I did not see them the day before (in all fairness, I wasn’t looking, but I think I would have noticed them). Did they just happen to land there before the storm passed?

Dr. Wikelski found that they only migrate in the day. What time of day did they land in my backyard?

Why would they choose this apparently far-from-optimal location to stop, if it wasn’t because of the weather?

4 Responses

Sorry, this crossed in the mail with a message I just sent you. Thanks for the kind words. I enjoy your blog immensely, and praise from you means quite a lot. Isn’t that stamp neat? I saw that, and I knew I just had to work it in, somehow.

Definitely! I’m thinking about buying one actually, and I’m thrilled that you shared your discovery of it. I had no idea they existed, but they are so symbolic of a field of study that I particularly love that I feel I must have them!